1. Field of the Invention
The present invention relates generally to secondary batteries, and more particularly to an improvement of a nonaqueous type secondary battery containing a carbon material as active material for a negative electrode and a nonaqueous electrolyte.
2. Description of the Background Art
Portable electronic or information equipment has been made greatly smaller or lighter in recent years, and secondary batteries for driving such portable equipment have been recognized as a critical part as a result. The lightweight lithium secondary battery with a high energy density which is regarded as a promising power source for driving portable equipment has been actively researched and studied. If a lithium metal is used as a negative electrode, however, repetition of charge/discharge cycles allows dendrites to grow on the lithium metal and an internal short circuit results. While use of a lithium alloy containing aluminum in place of a lithium metal has been proposed, repetition of charge/discharge cycles or deep charge/discharge causes segregation of the alloy, and sufficient battery characteristics may not be obtained.
It is why a battery containing a carbon material as a host material and having a negative electrode which takes advantage of intercalation and deintercalation reactions of lithium ions has been developed and reduced to practice. Such a lithium secondary battery using a carbon material for a negative electrode is excellent in terms of charge/discharge cycle characteristics and safety. However, carbon takes various forms from graphite to amorphous carbon, and the crystal structure or the microtexture of carbon materials greatly affect the performance of a resultant electrode, and therefore various carbon materials have been proposed as an electrode material.
Among such carbon materials, use of a graphitized carbon material provides a charge/discharge capacity approximate to a theoretical value, while the potential during charge/discharge is vary flat and very close to the potential at which lithium dissolves and precipitates, and therefore a battery with a high capacity and a sufficient charge/discharge potential may be implemented.
Such a graphite material which tends to cause decomposition of an electrolyte by its high crystalinity is known to be applicable as a negative electrode only if used with an electrolyte containing a particular organic solvent. If the negative electrode contains a graphite material, the kind of applicable electrolyte is limited, and an improvement of the temperature characteristics or charge/discharge cycle characteristics of the battery based on a selection of electrolyte is restricted to a great extent.
Propylene carbonate, for example, has good stability in oxidative decomposition and a low freezing point (-70.degree. C.), and is extensively used as a useful electrolyte for a lithium battery. It is reported however that if an electrolyte containing propylene carbonate is used with an electrode containing graphite, the presence of graphite intensifies the decomposition reaction of electrolyte and that charge/discharge for a graphite containing electrode cannot be carried out if the electrolyte contains only 10% propylene carbonate (see J. Electrochem. Soc., Vol. 142, 1995, pp. 1746-1752).
In order to solve such a problem, Japanese Patent Laying-Open No. 4-368778 proposes use of a carbon material produced by coating the surface of graphite particles with amorphous carbon as an electrode material. Such coated graphite particles reduce decomposition of an electrolyte, effectively increase the capacity of a battery and improve the charge/discharge cycle characteristic. On the other hand, propylene carbonate is very sensitive to the surface of graphite. In the manufacture of a battery using an electrolyte with propylene carbonate as a main component, as the coated graphite particles are ground or classified to have the same size in the manufacturing process or the graphite particles are kneaded with a binder or applied onto a current collector, if amorphous carbon coating the surface of graphite particles partly comes off, gas generated by decomposition of the electrolyte at the portion damages the electrode, resulting in a decrease in the capacity of the battery or degradation of the charge/discharge cycle characteristic. As a result, a battery having sufficiently high capacity cannot be manufactured and a low yield results. In order to solve such a problem it would be desirable to form an amorphous carbon coating layer with a sufficient thickness on graphite particles. As to this kind of carbon material, however, graphite with high crystallinity determines charge/discharge capacity. Increase in the ratio of amorphous carbon coating relative to graphite particles reduces the charge/discharge capacity, and therefore, improvement of the yield of batteries is achieved at the cost of capacity.
As a manufacturing method which would require a reduced manufacturing cost, there is a method of baking a mixture of a carbon precursor such as pitch and graphite particles. According to the method, since the procedure goes through the step with the liquid phase, graphite particles coated with less-crystallized carbon inevitably tend to stick with each other. The step of separating the coated graphite particles by grinding is necessary to control the thickness of a resultant electrode. As a result of such a step, the active surface of graphite particles is partly exposed which causes decomposition of propylene carbonate.